Triple negative breast cancer (TNBC) [estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) negative breast cancer is an aggressive subtype of breast cancer for which there are no approved targeted therapies. While standard chemotherapy reduces the risk of a disease event, patients with residual TNBC after neoadjuvant chemotherapy have a high risk of locoregional recurrence despite surgical resection and aggressive postoperative radiotherapy. Therefore, better understanding mechanisms of TNBC progression and identifying novel treatment approaches for patients who have progressed on standard treatment are of great needs. PD-L1 is overexpressed in TNBC, relative to normal breast tissue and other breast cancer subtypes. Aberrant PD-L1 expression on tumors is an important means of evading elimination by its host immune system. The binding of programmed death ligand 1 (PD-L1) to its receptor, programmed cell death protein 1 (PD-1) transmits signals that inhibit T- cell activation. Therefore, abrogating the PD-1/PD-L1 interaction with therapeutic antibodies has been explored as a means to enhance antitumor immunity. Although the extracellular role of PD-L1 in the regulation of T-cell responses has been well studied, potential intracellular functions of PD-L1 in cancer remain largely unknown. Surprisingly, we have found that TNBC proliferation requires PD-L1 and a subset of PD-L1 localizes in the nucleus and interacts with cohesin, a protein complex that is important for appropriate chromosome alignment and segregation during the cell cycle. Our Preliminary Data suggest that PD-L1 directly regulates cohesion function in TNBC. Knocking down PD-L1 dramatically causes incomplete chromosome segregation and inhibits TNBC cell proliferation, while has no effect on normal cells. The central hypothesis being tested in this proposal is that PD-L1 regulates cell cycle and chromosomal stability in triple negative breast cancer (TNBC), and targeting the intracellular/nuclear function of PD-L1 or pathways (mitosis and cohesin) regulated by PD-L1 is of therapeutic use. We propose to test this central hypothesis in the following Specific Aims: Aim 1, Determine the role of PD-L1 in regulation of cell cycle, genomic stability, and tumor cell proliferation by studying the exact mechanisms by which nuclear PD-L1 might regulate cohesion. Aim 2, Study the regulation of PD-L1 during cell cycle and mitosis. Aim 3, Evaluate the inhibition of PD-L1 nuclear function on chromosome segregation, tumor growth and response to radiochemotherapy both in vitro and in animal models. The overall impact from the successful completion of this work will be a more complete understanding of the role of PD-L1 in cancer pathogenesis. In addition, our work will lead to the design of more rational and effective combination therapies for TNBC patients by defining novel strategies that not only enhance cancer therapy by inhibiting mitosis but also unleash the antitumor activity of the patient?s immune system.